1. Field of the Invention
The invention relates to an image forming apparatus such as copying apparatus, printer, or the like and, more particularly, to a fixing device which is applied to such an image forming apparatus.
2. Related Background Art
Hitherto, in an image forming apparatus such as copying apparatus, laser beam printer, or the like using an electrophotographic method or an electrostatic recording method, in case of forming an image, first, an electrostatic latent image corresponding to an original image or an image signal which was inputted is formed on the surface of a photosensitive material or a dielectric material as an image holding member. The electrostatic latent image is developed as a toner image by developing means by using a toner having predetermined charges. The toner image developed on the image holding member is electrostatically transferred onto a recording material by transfer means. The toner image on the recording material is fixed onto the recording material by fixing means.
Hitherto, as a fixing method in such a kind of image forming apparatus, a heat roller method of conveying a recording material holding a toner image while sandwiching it by a heating roller and a pressurizing roller has been widely used. In such a heat roller method, since a heat capacity of the heating roller is large, there is a problem such that it takes a long time (what is called a warm-up time) which is required to heat the heating roller up to a predetermined temperature.
Therefore, a film heating fixing method whereby a warm-up time is reduced by using a thermal head of a low heat capacity and a thin film which moves in slide contact with the thermal head has been proposed (refer to Japanese Patent Laid-Open Application Nos. 63-313182, 2-157878, or the like).
As such a thermal head of a low heat capacity, a ceramic heater constructed in a manner such that a heat generating layer of a resistor member is provided on a substrate made of ceramic or the like and a protecting layer is further formed on the heat generating layer is generally used. Since the ceramic heater has a low heat capacity, it is possible to rapidly raise a temperature in a short time. On the other hand, when the temperature is raised, if an electric power is applied to the ceramic heater, there is a case where the heater is deformed by a stress due to a temperature difference in the heater. When rapidly raising the temperature with a low heat capacity, it is desired that a thickness of the heater is reduced as thin as possible and a large electric power is applied to the ceramic heater. However, in this case, there is a problem such that a possibility that the heater is damaged by a stress due to the foregoing temperature difference in the heater rises.